Method for forming silicide contacts

ABSTRACT

Contacts having different characteristics may be created by forming a first silicide layer over a first device region of a substrate, and then forming a second silicide layer over a second device region while simultaneously further forming the first silicide layer. A first contact hole may be formed in a dielectric layer over a first device region of a substrate. A silicide layer may then be formed in the first contact hole. A second contact hole may be formed after the first contact hole and silicide layer is formed. A second silicidation may then be performed in the first and second contact holes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority and benefitof Korean Patent Application 2005-0106099 filed on 7 Nov. 2005, theentire contents of which are hereby incorporated by reference.

BACKGROUND

Integrated circuit (IC) chips include millions of transistors and othercomponents fabricated on a substrate of semiconducting material such assilicon. The components are interconnected through lines of conductivematerial, most commonly aluminum. As IC chips increase in density andcomplexity, the dimensions of silicon features must be reduced in orderto pack more components onto a single chip. One technical challenge inreducing feature size is maintaining adequate electrical connectionsbetween silicon features and the conductive lines. As the feature sizedecreases, the contact area between the metal and the silicon decreases,and thus, the contact resistance increases.

To provide more reliable contacts through smaller surface areas, certainmetals such as refractory or near-noble metals may be used because theyreact with silicon to form silicides which create highly conductive andreliable contacts. A silicide contact is typically formed by depositinga thin layer of metal over silicon regions where contacts are to beformed, and then thermally treating it to cause the silicon and metal toreact and form a silicide layer between the metal and silicon. Unreactedmetal is removed, and the silicon features are then interconnectedthrough conventional conductive lines which now contact the silicidelayer rather than the silicon.

FIGS. 1-4 illustrate a prior art technique for forming silicidecontacts. Referring to FIG. 1, a metal-oxide-semiconductor (MOS)transistor having a gate electrode 10 and source/drain regions 30 (alsoreferred to as active regions) is first fabricated on a substrate 40 ina conventional manner. The gate electrode 10 includes a polysilicon gatelayer 11, and a mask layer 13 which are stacked on top of an insulatinglayer 15 that covers the channel region of the substrate between theactive regions 30. Spacers 20 are formed on the sidewalls of the gateelectrode 10 and are typically used during the manufacturing process toprovide accurate alignment of the active regions 30 with the sidewallsof the gate electrode 10. For example, the active regions 30 aretypically heavily doped with impurities so that they are highlyconductive. The sidewall spacers 20 facilitate the creation of lightlydoped drain (LDD) regions in the portions of the active regions 30 thatlie directly under the spacers. The LDDs tend to reduce the electricfield strength and provide more accurate control of the impurity dopingnear the edges of the gate 10, both of which tend to critically effectthe operation of the transistor.

Referring to FIG. 2, a dielectric layer 50 is deposited over thesubstrate, and then contact holes 51 are formed in the dielectric layer50 to expose portions of the active regions 30. A thin layer of metal 60is next deposited over the surface of the dielectric layer 50 as well asthe exposed portions of the active regions 30. The entire device is thenthermally treated to cause the metal at the bottom of the contact holes51 to react with the silicon to form a silicide layer 70 as shown inFIG. 3. Unreacted metal is then removed, at least on the top of thedielectric layer 50, and interconnects are formed by depositing a layerof aluminum 80 in the contact holes 51 as shown in FIG. 4.

The characteristics of the silicide layer 70, e.g., thickness,resistance, and the like, may be controlled through various parameterssuch as the type of metal, annealing time and temperature, etc.

One particular problem with formation of a silicide layer is lateralgrowth of the silicide along the surface of the substrate. For example,encroachment of silicide under the sidewall spacers and even into thechannel regions can result in increased leakage current and/ortransistor failure. One prior art technique for preventing unwantedsilicide encroachment involves implanting encroachment-inhibiting ionssuch as nitrogen into the active regions prior to thermal treatment.

Another problem is that certain transistors may be critical to theoperating speed of the IC, and therefore may require thicker silicidecontacts with lower resistance, while the vast majority of othertransistors are less critical and only require thinner silicidecontacts. A prior art technique provides thicker silicide layers forselect transistors by using lower impurity doping concentrations in theactive regions of the select transistors, while using higherconcentrations for the active regions of other transistors.

SUMMARY

Some inventive principles of this patent disclosure relate to methods inwhich a first silicide layer is formed on a first device region of asubstrate, and a second silicide layer is formed on a second deviceregion while simultaneously further forming the first silicide layer,which may accommodate different contact areas for the first and secondsilicide layers. Other inventive principles of this patent disclosurerelate to methods in which first and second contact holes are formed ina dielectric layer over first and second device regions of a substrate.The second contact hole may be formed after the first contact hole isformed and a silicide layer formed therein. A second silicidation maythen be performed in the first and second contact holes. Additionalinventive principles of this patent disclosure relate to semiconductordevices having first and second silicide layers with substantiallydifferent thicknesses in first and second device regions havingdifferent pattern densities. The first and second silicide layers may beformed over first and second active regions, respectively, that may beimplanted with impurities through the same implantation process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 illustrate a prior art technique for forming silicidecontacts.

FIGS. 5-12 illustrate an embodiment of a semiconductor device and anembodiment of a method for forming silicide contacts according to theinventive principles of this patent disclosure.

DETAILED DESCRIPTION

As the density and complexity of IC chips continue to increase, the typeand density of various silicon features become more diverse, and thus,the size and shape requirements of contacts for these features may varygreatly. For example, a first region of a substrate may be fabricatedwith lower density gate patterns, and thus, more space for contactsbetween the gates than a second region. If the same process is used toform a silicide layer for both larger contacts in the first region andsmaller contacts in the second region, it may result in eitherinsufficient silicidation of the larger contacts or silicideencroachment around the smaller contacts.

FIGS. 5-12 illustrate an embodiment of a semiconductor device and anembodiment of a method for forming silicide contacts according to theinventive principles of this patent disclosure. FIG. 5 is across-sectional view of a semiconductor wafer including a substrate 101having first and second device regions I and II. Device region Iincludes a relatively low density pattern of features, e.g., gateelectrodes, whereas device region II includes a high density pattern offeatures. Transistors in the first device region I include active(source/drain) regions 130, gate electrodes 110 (which, in turn, includea gate layer 111 and a first hard mask layer 113), and sidewall spacers120. Transistors in the second device region II include active regions230, gate electrodes 210 (which include a gate layer 211 and a secondhard mask layer 213), and sidewall spacers 220.

An interlayer dielectric layer, for example, an oxide layer 310,320 isdeposited over both device regions I, II as shown in FIG. 6. Normalcontact holes 410 are formed in the dielectric layer 310 to exposecontact areas of the active regions 130 as shown in FIG. 7.

Referring to FIG. 8, a first metal layer 420 of, for example, arefractory metal such as titanium (Ti) is deposed over the substrate 101and covers the dielectric layer 310,320 and the exposed portions of theactive regions 130. The metal may be deposited by chemical vapordeposition (CVD) or any other suitable process. In one embodiment, awafer having the dielectric layer 310,320 with contact holes 410 overthe first device region I may be loaded into a chamber which isevacuated and then supplied with processing gases including titaniumtetrahalide (TiCl₄), hydrogen (H₂), and argon (Ar). A plasma is producedin the chamber to deposit a titanium layer in the contact holes at atemperature that is preferably between about 400° C. and about 800° C.The titanium layer may react with silicon in the exposed portions of theactive regions simultaneously as it is being deposited, forming a firstsilicide layer 430, in this example, titanium-silicide. Other metals maybe used, for example cobalt, Nickel, etc. to form other silicides suchas CoSi₂ or NiSi₂. At this point, the first silicide layer 430 is formedto a first thickness that may be insufficient to provide an appropriateamount of contact resistance for the devices formed in the first deviceregion I.

Referring to FIG. 9, unreacted metal is removed through selectiveetching, leaving the silicide layer 430 at the bottoms of the contactholes 410. In one embodiment, immediately after the simultaneous depositand silicidation of the metal film, unreacted metal may be removed byexposure to a flow of titanium tetrahalide (TiCl₄) which etches awayexposed metallic titanium, but does not etch the silicide (TiSi₂) on theactive regions. The etching may be performed by flowing the same gasesas during the deposition process, but with the plasma power off.

Referring to FIG. 10, second contact holes 510 are formed in thedielectric layer 320 to expose contact areas of the active regions 230in the second device region II. In this example embodiment, the contactholes 510 are self-aligned contact holes (e.g., defined by the sidewallspacers 220) as opposed to the normal contact holes (e.g., formedthrough conventional photolithography techniques) 410 of the firstdevice region I. The self-aligned contact holes 510 may be used becauseof the closer spacing of gate electrodes 110. Thus, the contact areas onthe active regions 230 are generally smaller than those in region I, asare the silicon features, e.g., active regions.

As shown in FIG. 11, a second metal layer 520, again Ti as an example,is deposited on the first silicide layer 430 in the normal contact holes410, as well as the exposed contact areas of the active regions 230 inthe self-aligned contact holes 510. The metal formed on the firstsilicide layer 430 further forms the silicide layer to a greaterthickness as shown at 435 in FIG. 11. Metal deposited in theself-aligned contact holes 510 reacts with silicon in the active regions230 to form a second silicide layer 535 on the second device region II.The second silicide layer 535 may be thinner than the further grownfirst layer 435. Thus, the inventive principles of this patentdisclosure may enable the formation of a silicide layer in the seconddevice region II having different characteristics than the silicidelayer in the first region I to accommodate devices having differentpattern densities, device spacing, contact areas, etc.

Referring to FIG. 12, unreacted metal may be removed from the top of thedielectric layer 310,320, and then metal plugs 440 and 540 are formed byfilling in the normal and self-aligned contact holes 410 and 510respectively, with a metal such as tungsten (W), copper (Cu), oraluminum (Al).

Another aspect of the inventive principles of this patent disclosure isthe creation of a semiconductor structure having active regions withsubstantially equal doping concentrations but with substantiallydifferent silicide layers formed thereon. For example, in theembodiments of FIGS. 11 and 12 a structure may be created where activeregions 130 and 230 in different device regions I and II may beimplanted with impurities through the same implantation process, butthen connected through contacts that have different silicidethicknesses.

The embodiments described above can be modified in arrangement anddetail in accordance with the inventive concepts of this patentdisclosure. For example, embodiments have been described above usingtitanium (Ti) to form silicides, but other refractory metals, near-noblemetals, etc. may be used. Likewise, contact areas have been described asbeing defined by specific types of contact holes on specific types ofsilicon features such as source/drain regions, but the inventiveprinciples of this patent disclosure are not limited to any specifictypes of contacts, contact holes, silicon features, etc. As a furtherexample, different numbers of silicide processes maybe used to formcontacts in more than two device regions. For example, one silicideprocess in a first region, two processes in a second region, and threeprocesses in a third region. Therefore, such changes and modificationsare considered to fall within the scope of the following claims.

1. A method comprising: forming a dielectric layer over a substrate;forming a first contact hole in the dielectric layer over a first deviceregion of a semiconductor substrate to expose a first source/drainregion; performing a first silicidation in the exposed firstsource/drain region; forming a second contact hole in the dielectriclayer over a second device region of the substrate to expose a secondsource/drain region after performing the first silicidation; andperforming a second silicidation in the exposed first and secondsource/drain regions.
 2. The method of claim 1 where the first contacthole is a normal contact hole.
 3. The method of claim 1 where the secondcontact hole is a self-aligned contact hole.
 4. The method of claim 1where the second silicidation is performed simultaneously in the firstand second contact holes.
 5. The method of claim 1 where: the firstcontact hole is formed between devices having a first pattern density;and the second contact hole is formed between devices having a secondpattern density greater than the first pattern density.
 6. The method ofclaim 1, wherein the first and second source/drain regions are implantedwith impurities through the same implantation process.
 7. The method ofclaim 1, wherein a silicide layer formed in the first contact hole afterthe second silicidation is thicker than a silicide layer formed in thesecond contact hole after the second silicidation.
 8. A methodcomprising: forming a contact in a first contact hole over a firstregion of a substrate using a first number of silicidation processes;and forming a contact in a second contact hole over a second region ofthe substrate using a second number of silicidation processes less thanthe first number; wherein during the silicidation processes silicon inthe substrate is reacted with metals.
 9. The method of claim 8 furthercomprising forming a dielectric layer over the first and second regions.10. The method of claim 9 further comprising: forming the first contacthole in the dielectric layer over the first region; and performing oneof the silicidation processes in the first contact hole while the secondregion is masked by the dielectric layer.
 11. The method of claim 10further comprising: forming the second contact hole in the dielectriclayer over the second region; and performing one of the silicidationprocesses in the first and second contact holes simultaneously.
 12. Themethod of claim 8 where: the first contact hole is a normal contact holeformed between devices having a first device spacing; and the secondcontact hole is a self-aligned contact hole formed between deviceshaving a second device spacing closer than the first device spacing. 13.The method of claim 8 further comprising: forming a contact over a thirdregion of the substrate using a third number of silicidation processeswhere the third number is less than the second number.
 14. The method ofclaim 8 where: the contact in the first contact hole is silicided usingone metal layer; and the contact in the second contact hole is silicidedusing two metal layers.
 15. The method of claim 8, wherein the first andsecond source/drain regions are implanted with impurities through thesame implantation process.
 16. The method of claim 8, wherein a silicidelayer formed in the first contact hole using the first number ofsilicidation processes is thicker than a silicide layer formed in thesecond contact hole using the second number of silicidation processes.